Current practice in obtaining high liquid yields from the thermal decomposition of biomass at short reaction times (sometimes called "fast", "flash" or "rapid" pyrolysis) makes use of reactor types capable of high heat transfer rates to small biomass particles, in order to achieve the rapid heat-up rates necessary. Three of the most commonly used types are the fluidized bed, the circulating fluidized bed or the transport reactor. In the first two of these, hot gases and solids, normally inert, are brought into intimate contact with the biomass particles. In transport reactors, either hot gas alone or a mixture of hot gas and solids may be used. All of these reactors have in common a requirement for a significant gas flow, usually from 1 to 10 times the weight of biomass being processed. If pyrolysis is carried out in the absence of oxygen, then the non-condensable gases formed will have significant contents of carbon monoxide, hydrogen, methane, and other light hydrocarbons or organics, and are of medium calorific value and can be readily burned in air.
In order to preserve the high calorific value of these non-condensable gases, and to prevent the loss of organic liquid yield due to uncontrolled oxidation reactions if air is present, it is current practice to use these gases as a recycle stream to supply the necessary fluidizing or conveying gas for reactor operation. It is also current practice to heat this recycle stream indirectly in order to supply part or all of the heat necessary for the pyrolysis reaction.
The operation of a fluidized bed process has been described by Scott and Piskorz (1) (2). When poplar wood was used as feed in a fluidized bed of sand at a temperature of 500.degree. C. and a gas apparent residence time of 0.48 seconds at a gas to-feed weight ratio of 3:1, the recycle gas composition, on a moisture-free basis, was:
______________________________________ Hydrogen 1.49% by volume Carbon monoxide 47.83% Carbon dioxide 39.40% Methane 6.97% Ethylene, ethane, etc. 4.31% ______________________________________
The yield of gas was 11.1%, of organic liquid 66.3%, and of char 11.8%, expressed as weight % of the moisture-free feed. The balance of 10.8% was water formed in the pyrolysis reactions.
It is apparent that a gas with such a high concentration of carbon monoxide would be extremely toxic, and even small leakages or emissions would pose a severe hazard to life. In addition, the gas can readily form explosive mixtures with air due not only to the carbon monoxide content but also due to the content of other inflammable hydrocarbons and hydrogen. However, in the prior art as described in these publications and also in our earlier Canadian Patent No. 1,241,541 (September, 1988), it is specified that the gas used must be oxygen-free.
Pyrolysis with a transport reactor in which both gas and hot solids were mixed and transported with the biomass is described by Graham et al (3). Poplar wood was pyrolysed at 650.degree. C. and 0.524 seconds apparent residence time. Although gas recycle was not used in the reported experiments (nitrogen was used), if gas recycle had been practised as would be expected in a larger scale unit, then, on an inerts-free basis, this reactor would give a gas for the above conditions having the following analysis:
______________________________________ Hydrogen 4.18% by volume Carbon monoxide 63.10% Carbon dioxide 11.80% Methane 12.55% Ethylene, ethane etc. 8.37% ______________________________________
Clearly, a gas of this composition would be hazardous in practice, being both highly toxic and readily forming explosive mixtures with air. In this work, the carrier gas used was inert. In subsequent disclosures by Underwood and Graham (U.S. Pat. No. 4,876,108, Oct. 24, 1989 and U.S. Pat. No. 4,994,297, Feb. 19, 1991), three reactor systems were described for the preparation of fast pyrolysis liquids from wood or cellulose (4). In every example, and in all claims , it is specified that the process is to be carried out in the absence of oxygen. For the RTP unit described, it is specified that inert gas is to be used together with suspended particulate solids.
It is highly desirable to make use of the combustible non-condensable gases produced during pyrolysis in order to recover their heating value and thereby improve the thermal efficiency of the process. If this is done by use of a recycle stream of product gas after venting the pyrolysis product gas for use as a fuel supplement, then significantly large amounts of such gases must be cooled and heated to fulfill their function as carrier or fluidizing gases for the reactor used. Given the highly toxic and explosive nature of this recycle gas stream, it will be necessary in practice to build into the process extensive alarm systems, and emergency ventilation and fire prevention equipment at considerable capital cost. A high degree of automation may be found to be required. Environmental considerations may add additional capital and operating costs. It is particularly difficult to prevent escape of some process gas at the point where biomass particles, must be fed to the system. Even if lock hopper feeders or other positive devices are used, an extensive purging capability must be installed.